1. Field of the Invention
The present invention relates to the field of image scanning, particularly scanning of images by coherent radiation, particularly essentially monochromatic coherent radiation and more particularly optical scanning of images with a mechanical system. The invention relates to such optical/mechanical scanning systems which provide sufficiently high resolution as to enable the scanned image to be used in photolithographic manufacture of printed circuit boards (PCB) and printing plates.
2. Background of the Art
Optical scanning techniques have found important applications in many technical fields. These fields include, for example, laser printers for computers; laser direct writing lithography for production of masks, wafers and optical integrated circuits; high speed photography; IR imaging; image information transmission; thermal and mass transfer of images; graphic arts imaging for newspapers and other printed materials; direct imaging for printed circuit board fabrication; and the like. Many different scanning technologies, software, hardware and qualities are available commercially and in the literature. A summary of scanning methods is presented in the articles by Leo Beiser (Laser Focus/Electro-Optics, Feb. 1985) and Henry E. R. Lassiter (Laser Focus World, Jan. 1991).
A significant challenge in the generation of scanned data particularly occurs with respect to the provision of large format image recording systems. One aspect of the technical problems faced in producing high resolution image data within the large format field is the technically contradictory objectives to record a large image area, yet use the smallest possible pixel size in the generated data while still providing the data in the shortest time or at least a reasonable time. The issues tend to be contradictory as it takes more data and more time to provide image data when using small pixels. When working with a fixed or standard large area format image, the greater the resolution, longer exposure times are needed to generate the data with high resolution. For example, with a typical plate size in Printed Circuit Board applications of 20" by 24" (50.8 cm.times.61 cm), the total number of binary pixels is about 256.times.10.sup.6 pixels for 5080 (2000 dots per centimeter) dot per inch resolution. If this image were to be recorded in 1 minute, it would require a sustained data generation rate of about 0.5 Gbit/sec. Such high rates and high resolution in scanned data systems pose a significant challenge to imaging systems that are presently under development or recently available, and these high rates and high resolutions have not yet been met satisfactorily with commercial equipment.
Existing optical scanning systems tend to be limited in data rate acquisition by mechanical speed limitations, light switching speed limitations or both. For example, X-Y stage-based scanning systems have relatively low imaging rates which are limited by the low translation speed of the stages. External or internal drum scanning systems, on the other hand, may achieve medium to high imaging speed rates, but are limited to exposing flexible materials because of the physical construction of the system. Images on non-flexible surfaces must be converted to a flexible surface if the images are to be used on a drum system. This conversion to an intermediate image for use on the drum system is itself likely to reduce the ultimate resolution of the final image by introduction of errors in the formation of the intermediate image. Even when software is used to compensate for variations in image data received on a curved or drum surface, there will still be some additional loss of resolution from the process. Serial scanning systems, i.e., systems in which a single channel is used throughout the data path and light modulating to record an image, are limited by their light switching speed, since they require a light modulation switching speed which is the same as the incoming data rate.
Furthermore, polygon based scanning systems in which the scanning trace is a straight line (or any other type of scanning system having a single channel, a serial scanning system) are limited in format size, since the entire area of the format must be covered by the scanning field of view. When an angular or deflected scanning beam is used to attempt to cover a wider area than would be provided by a non-varying straight line polygon scanning system, additional resolution issues are introduced into the data acquisition system. A common compromise in such systems is to make a trade off between the spot size for field of view and writing energy. U.S. Pat. No. 5,216,247 (Wang et al) is a representative example of such tradeoff. It offers relatively high speed scanning and a potential for relatively higher resolution.
Unfortunately, such an optical scanning system, by its very nature, needs to work with a very low numerical aperture, severely limiting the spot size used and the exposure energy capability of the system. For an image format of 22 inches by 28 inches (55.9 cm.times.71.1 cm), circles as large as 60 inches (152.4 cm) need to be scanned to avoid scan line distortion. For this, a minimum working distance of 30 inches (76.2 cm) would be required to keep the field of view below 90 degrees. This working distance makes the achievement of a numerical aperture (NA) above 0.2 very complex and expensive. This limitation on the NA also limits the spot size. Additionally, this prior art is still limited by light switching speed. It is a serial scanner by its nature and construction, and it must be switched by a single light switch that would need to possess the switching speed of the system or, in other words, the data rate. As previously mentioned, this is a significant issue in the development of a commercially viable system.
The same type of limitation exists in linear scanning systems, which are built around a flatbed substrate carrier. Although flatbed scanners may feature perfectly orthogonal scanning systems, such systems must also compromise between the numerical aperture (and, hence, spot size and exposure energy) and the speed desired for large format imaging. In one type of implementation, where the scan covers part of the image, the imaging speed is limited by the x-y stage direction change. This forces the use of a low NA, large spot and low exposure energy density.
Another limitation associated with existing rotational scanning systems, such as shown in FIG. 3, is the inherent distortion associated with the rotational scanning method. In such systems, an exposure head is rotated at the same time that the working plane is translated linearly to facilitate complete coverage of the image. This mode of scanning can result in incomplete overlap of consecutive swaths of the exposure head, as shown in FIG. 4, unless the linear movement is so incremental as to force the scans to overlap. This, however, directly increases the time needed to scan a given area since the number of scans along the line of linear movement must be increased. As shown in FIG. 4, the scan pattern transcribes, in a single pass, concentric outer radius R1 and inner radius R2. A subsequent scan transcribes the same pattern, but is linearly offset by the translation of the stage. It is apparent that the two consecutive scans may not perfectly overlap, as they are not concentric. It is impossible to compensate for such lack of overlap unless the incremental distance of movement between each scan is significantly less than the width of the scanning spot, resulting in image distortion.
A review of rotational scanning systems recording image on planar surface is given in U.S. Pat. No. 5,216,247 by Wang et al.
Various scanner systems known in the literature include the following, such as U.S. Pat. No. 3,588,218 which describes a multiple spot scanner with an optical relay system in which at least one optical beam is periodically refocused to define a plurality of scanning light spots along a common scan locus. This system uses a drum as the scan locus in which the focused beam forming the spots is returned to a rotating drum after each spot formation.
U.S. Pat. No. 4,301,374 describes a shutter system for an optical multi-lens scanner. The readout uses a laser light source and focusing optics to direct the light onto the data record for modulation and subsequent readout of the modulated light. The shutter elements are interposed to intercept the transmission of light in response to control signals in the data. As can be seen in FIG. 1, the locus of lens travel is arcuate.
U.S. Pat. No. 3,704,372 describes an optical pattern line/edge tracer which uses a motor driven mirror to produce a rotary scan. A mirror assembly is exchangeable to provide varying scan diameters. Reference signals are sampled instantaneously to produce coordinate drive signals. The tracing apparatus appears to be limited to scanning systems arranged to scan a circular path on a surface bearing an image. The mirror is rotated by a motor causes a photocell to effectively scan a surface in a circular path.
U.S. Pat. No. 4,413,180 describes an apparatus and method for image acquisition in which a conical beam of light is generated by impinging a light beam on a surface of a concave, cylindrical reflector. A light beam enters the conical, cylindrical reflector through a hollow shaft of a motor. An illuminated portion of an object is imaged on an array of photosensitive elements to produce corresponding signals that are representative of the image on the array of photosensitive elements. A conical beam is produced which may be used in a robotic mapping function.
U.S. Pat. No. 4,611,811 describes an optical apparatus for scanning radiation over a surface. The apparatus includes a plurality of optically spaced rotating arms extending from a central axis. A central reflector is positioned at the axis of the arms, and a radially remote reflector is positioned at the axis of the arms. The central reflector rotates with the arms but is sequentially indexed into optical path alignment with each radially remote reflector so that a beam of radiation in the optical path is scanned over the same predetermined arcuate segment by each of the three remote reflectors. The axial reflectors do not need to be positioned equidistant from the axis of rotation of the arms.